(1) Field of the Invention
The present invention relates to electrically conductive polymer blends containing a thermoplastic and an intrinsically conductive polymer, and more particularly to electrically conductive polymer blends containing a thermoplastic and an intrinsically conductive polymer that have a low percolation threshold and methods for preparation thereof.
(2) Description of the Related Art
Electrically conductive plastics have promising potential for use in a growing number of commercially important applications. Fibers and films that are electrical conductors or semi-conductors can be used in anti-static fibers and coatings, corrosion resistant paints and coatings, radiation absorbing materials, electrical components such as capacitors, electrodes and battery components and in many other useful articles.
Basically, there are two approaches for producing conductive plastics; one approach involves blending insulating polymers with electrically conductive particles such as carbon black, metal fines or other conductive non-polymeric material. The other approach involves the synthesis of intrinsically conductive polymers (ICP's) such as polyaniline, polypyrrole, polythiophene, or the like, which are polymers that are capable of charge transfer along a system of conjugated double bonds arranged along the polymer backbone.
The use of conductive particulates to increase the conductivity of a polymer is well known in the art and has provided successful products for many applications. However, particulates are rarely compatible with the matrix polymer and very high loading levels are often required to provide electrically conductive pathways through the matrix polymer. The formation of these pathways is referred to as "percolation", and the level of particulate, or conductive additive, at which such pathways are first formed is termed the "percolation point" or "percolation threshold". The critical concentration for percolation is theoretically associated with the point of steepest rise in a plot of the logarithm of conductivity vs. the percentage of loading volume of the conductive material. Particulates that require a high percolation threshold can bleed out of the matrix polymer and contaminate surrounding materials. Moreover, the use of high levels of particulate material often results in a significant reduction in the mechanical strength of the host polymer, making it less suitable for fibers, films, or other applications that require good tensile properties.
Because ICP's are conductive without the addition of such particulates, they potentially have advantages in applications where optical quality of the polymer is important, or where particle-filled polymers can not provide needed levels of conductivity or strength. As used herein, the term "ICP" is meant to include any polymer having a conjugated .pi. electron system that is electrically conductive in at least one valence state. It is well known that some ICP's, such as polyaniline, may be reversibly made conductive by the addition or removal of a protonic acid dopant. Addition of an acid dopant to polyaniline, for example, forms the conductive polyaniline salt, while removal of the acid results in the non-conductive base form. When an ICP is referred to as being "conductive", it is meant that it has an electrical conductivity of at least 10.sup.-8 S/cm. An ICP is "non-conductive" if it has an electrical conductivity of less than 10.sup.-8 S/cm.
Despite several advantages over filled polymers, the commercial application of ICP's has been held back because they are characteristically difficult to process due to very limited solubility and lack of fusibility. Also, films and fibers of pure ICP homopolymers are often brittle and lack the tensile properties necessary for forming textiles or rugged coatings. Furthermore, most ICP's are significantly more expensive than conventional commercial bulk thermoplastic polymers.
Many approaches have been tried to improve the mechanical and processing qualities of ICP's while retaining a desirable level of conductivity. An early approach was to form polymer blends of ICP's with insulating thermoplastics in order to obtain materials having tensile properties of the thermoplastic and conductivity of the ICP. However, such materials often had high percolation thresholds and suitable conductivity was not obtained until ICP levels of 15% to 25% or more had been incorporated into the thermoplastic. See, e.g., Andreatta and Smith, Synth. Met., 55-57:1017, 1993.
Blends of polyaniline with insulating thermoplastics were reported by Kulkarni et al., U.S. Pat. No. 5,217,649, wherein an ester-free plasticizer and an acidic surfactant were used to aid in formation of the blend. However, the composition showing the highest conductivity at 5% wt/wt polyaniline in the blend demonstrated a conductivity of only about 2.7.times.10.sup.-7 S/cm, which is too low for many anti-static applications.
Karna et al. EP 582,919 A2 and Karna et al., U.S. Pat. No. 5,340,499, reported that a conductivity of almost 0.1 S/cm could be obtained in a blend of acrylonitrile-butadiene-styrene (ABS) containing only 2% wt/wt of polyaniline, but only with the inclusion of high levels of a proprietary metal/organic acid dopant-plasticizer. Polyaniline blends were made with polystyrene, polypropylene and polyethylene and, while the blends showed reduced percolation thresholds over earlier blends, all had high levels of the metal/organic acid dopant complex. The requirement for such high levels of this expensive dopant would increase the cost for such blends.
Another approach to reduce the amount of ICP necessary to give a polymer blend a certain level of conductivity has been to produce materials having the ICP concentrated in a network, or in highly conductive fibrils, to provide a highly conductive pathway for charge transfer through the polyblend.
Wessling, in U.S. Pat. No. 4,929,388, taught the formation of conductive polymer blends containing two partially compatible thermoplastic polymers. An electrically conductive substance, such as carbon black, was loaded into one of the polymers, and the polymers were blended at increased temperature. It was found that if the two polymers had different melt viscosities and different solubility parameters, the polymer having the lower melt viscosity would form a continuous phase through the blend. Carbon black, loaded into the continuous phase polymer of a polyblend was believed to form a conductive network. The reference also reported that ICP's could be used as the conductive substance But, it was said to be preferable to add a crosslinking agent to the blend to physically stabilize the structure of the conductive network.
The production of blends of thermoplastics with ICP's that were formed by mixing solutions of the two polymers in different solvents was reported by Yang et al., in Synth. Met., 53:293-301, 1993. A structure in which the ICP formed a network through the thermoplastic could be produced by controlling the relative miscibility of the respective solvents. Electrical conductivity at an ICP level of under 1% was reported, but the items fabricated were limited to films from which the solvents could be easily evaporated.
Shacklette et al., in Synth. Met., 55-57:3532, 1993, reported percolation thresholds of about 5%-6% by volume for polyaniline (Versicon.RTM.) in polycaprolactone or poly(ethyleneterephthalate glycol) blends. They reasoned that these percolation thresholds were lower than that predicted theoretically for a dispersion of small spheres in a matrix polymer (15%-30% by vol.) because the lack of compatibility between the polyaniline and the matrix polymer caused the dispersed polyaniline particles to reagglomerate to form one and two dimensional aggregated structures of high conductivity through the bulk matrix polymer. However, no methods for obtaining predictable formation or maintenance of such structures was disclosed or suggested.
Conn et al., WO 96/21694, reported on the production of composite materials from insulating thermoplastic polymer particles which had been coated with conducting polymers. The coated particles were then thermally bonded into a composite in which the conducting polymer formed a conductive network through the composite. The composites were reported to have conductivities of up to about 30 S/cm and percolation thresholds of less than 1% of the conducting polymer. However, production of commercially useful fibers or films from such composites does not appear promising since the mechanical properties of the conducting composites were significantly reduced from those of the parent thermoplastic polymer. In addition, mixing of the thermoplastic at a temperature exceeding its flowpoint resulted in destruction of the network morphology of the conducting polymer.
Han et al., U.S. Pat. No. 5,378,404, described a method for forming dispersions of ICP's in a matrix polymer wherein an ICP that is incompatible with the matrix polymer is selected. As used in the patent, "incompatible" meant that the ICP and the matrix polymer had different solubility parameters and surface energies; were apt to be chemically reactive with each other; or had mismatched dispersive, polar or hydrogen bonding interactions. The ICP was then doped with a dopant acid which made the ICP more compatible with the matrix polymer. The doped ICP was blended with the matrix polymer to give an electrically conductive blend. The percolation point in a blend with polycaprolactone was about 8% and was from about 6% to 25% in blends with poly(ethyleneterephthalate glycol).
It has also been suggested by Shacklette that conductive blends could be made from a polar ICP (e.g., Versicon.RTM.) mixed with a partially compatible polymer such as ethyl vinyl acetate and that the percolation threshold of such blends could be reduced further by the addition of polar plasticizers.
Heeger et al., U.S. Pat. No. 5,491,027, produced microfibrils containing high levels of ICP's and then blended the microfibrils into polyethylene. Although the microfibrils themselves had high conductivities, the conductivity of the blends, even at about 20% ICP content, was around 10.sup.-8 to 10.sup.-10 S/cm. Later, Passiniemi et al., in Synth. Met., 84:775-776, 1997, reported that fibers were melt spun from a blend of polypropylene and polyaniline doped with a proprietary dopant/plasticizer. The fibers had conductivities of about 10.sup.-3 S/cm. The polyaniline was reported to have formed discrete fibrils within the polypropylene fiber matrix and the fibrils were mainly oriented along the fiber direction.
Tanner et al., in Synth. Met., 84:763-764, 1997, reported blends of polypropylene with doped polyaniline to which an ampiphilic plasticizer had been added. It was thought that the use of an ampiphilic dopant, such as dodecylbenzenesulfonic acid and an ampiphilic plasticizer resulted in the formation of a network structure in which the polyaniline formed a continuous phase if the polyaniline phase had a lower melt viscosity than the matrix polymer and that the matrix polymer be nonpolar in nature. However, the conductivity of such blends was about 10.sup.-3 S/cm at 15% polyaniline complex in the blend and the percolation point was not given. Plasticizers were also used in blends of polyaniline and cellulose acetate by Pron et al. as reported in J. Appl. Polym. Sci., 63:971-977, 1997. A blend of doped polyaniline, a plasticizer composition containing dimethyl phthalate, diethyl phthalate and triphenyl phosphate, and cellulose acetate was formed as a film deposited from a solution in m-cresol. The film was reported to have a percolation threshold of 0.05% based on the weight of emeraldine base in the blend. This low percolation point was reported to be due to a unidimensional aggregation of polyaniline grains in the blend. Such fabrication techniques, however, would be limited to thin films and other thin materials that could be formed by solvent evaporation.
Despite the promising results that have been obtained by the use of network structures in polymer blends to reduce the level of ICP necessary for a given level of conductivity, some problems still remain before such compositions find broad commercial success. In particular, methods must be found that result in stable and reproducible network structures having low percolation thresholds while maintaining commercially useful tensile properties.
It would, therefore, be advantageous to provide a blend of a thermoplastic and an ICP where the blend has a low percolation threshold and significant electrical conductivity at low levels of the ICP. Furthermore, it would be an advantage to provide a simple method for producing such polymer blends having a low percolation threshold. Moreover, it would be advantageous to provide such thermoplastic polymer/ICP blends that have mechanical properties that are suitable for applications such as textile fibers and to provide blends that can be processed by conventional processing methods into articles of a variety of shapes and sizes not limited to thin films or the like.